1. Field of the Invention
This invention relates to an apparatus for comparing frequency of an object signal, including the case of a frequency-modulated signal where the frequency is understood to be center, i.e., mean frequency of such signal; with a certain frequency.
2. Description of the Prior Art
For such comparing, it is conventional to use a frequency discriminator with calibrated chart or diagram showing the relation between the input signal frequency and the output voltage, low-pass filter or integrating circuit in case the object signal is frequency-modulated requiring averaging the discriminator output, a standard voltage generator for providing the voltage smae as the discriminator output corresponding to the said certain frequency according to the relation as given as above, and a comparator for comparing the averaged discriminator output voltage for the object signal with this standard voltage. The standard voltage generator may as well be constructed with a stable-frequency oscilator incorporating a crystal of the resonance frequency identical with said predetermined frequency and a discriminator with the same characteristics of frequency to output relation just as the discriminator already mentioned.
However, such apparatus has the fundamental defect in that the frequency to output voltage relation of the discriminator is not constant but may fluctuate in non-negligible manner as the temperature or humidity varies and/or even simply as the time lapses, of course in non-uniform manner one discriminator with another, and that the result of comparison is inevitably inaccurate because of such fluctuation.